Helicopters are known which substantially comprise a fuselage; a main rotor on top of the fuselage, and which rotates about a respective axis; a tail rotor at the rear end of the fuselage; two horizontal tail surfaces; and two vertical tail surfaces.
More specifically, the rotor substantially comprises a hub, which rotates about said axis and has a number of blades fixed to and projecting radially from the hub; and a drive shaft connectable to a drive member and connected functionally to the hub to rotate it.
Operation of the rotor produces high- and low-frequency vibration. More specifically, low-frequency vibration is produced by the wash from the blades and from the centre of the hub. The wash from the centre of the hub impacts the vertical and horizontal aerodynamic tail surfaces and the tail rotor.
To avoid generating low-frequency vibration, the rotor comprises a flow diverter mounted over the centre of the rotor and extending annularly about the rotor rotation axis.
More specifically, the flow diverter is designed to direct the wash produced by the rotor, so as to avoid the wash effects and prevent the wash from impacting the tail rotor and relative supporting structures.
High-speed rotation of the blades also produces high-frequency vibration, which is transmitted to the drive shaft and, hence, to the helicopter.
Generation and transmission of high-frequency vibration to the drive shaft of the rotor are prevented using damping devices fitted to the rotor and tuned to one or more high-frequency vibration frequencies.
One such damping device is described in Patent Applications GB-A-2014099 and FR-A-2749901.
In particular, FR-A-2749901 disclose a damping device, which reduces the transmission of vibrations in a plane orthogonal to the axis of the rotor to the fuselage. The damping device comprises driving means which bring a mass in a required adjustment position along the axis of the rotor, without exerting any damping action on this mass.
As a result, the damping device shown in FR-A-2749901 can in no way reduce the transmission of axial vibrations from the blades to the fuselage.
The Applicant's Patent Application PCTIB2008001594 describes a simple, low-cost damping device designed to prevent generation and transmission of high-frequency vibration to the drive shaft of the rotor, without interfering with the aerodynamics and operation of the rotor and/or flow diverter.
More specifically, the above damping device substantially comprises:                a mass housed in the flow diverter; and        a rod, which is supported coaxially by the shaft at a first axial end, and is connected to the mass at a second axial end opposite the first end.        
More specifically, the rod is axially rigid enough to secure the mass in a substantially fixed position along the rotor axis.
Conversely, the flexural rigidity of the rod is such as to allow the mass to vibrate in a plane perpendicular to the rotor axis and at a characteristic rotor rotation frequency, and therefore to oppose transmission to the shaft of flexural vibration produced by rotation of the hub and blades.
The damping device described above therefore only effectively opposes transmission to the shaft of flexural vibration in a plane substantially perpendicular to the rotor axis, and at a frequency around a specific given frequency determined by the flexural rigidity of the rod and the weight of the mass.
In other words, the damping device described above is a ‘passive’ element tuned to a specific frequency of the flexural vibration for damping.
A need is felt within the industry to effectively also oppose transmission to the shaft of axial vibration, i.e. parallel to the shaft axis.
Following the introduction on helicopters of variable-speed rotors, i.e. designed to rotate at different speeds during operation of the helicopter, a need is also felt to prevent transmission to the drive shaft of vibration with a highly variable frequency range.